This study investigates an expanded paradigm to improve health outcomes for adults with knee osteoarthritis (OA), through changes in the low end of the physical activity spectrum where much of the knee OA population resides. Our recent study found almost half of adults with knee OA were inactive, as defined by the U.S. Department of Health and Human Services (DHHS) (i.e., no sustained 10 minute periods of moderate-to-vigorous intensity physical activity during a week) based on objective accelerometer measurement of physical activity. It is these inactive people who will be the primary focus of this study. This study will compare health outcomes of inactive adults at baseline who become active two years later versus inactive adults at baseline who remain inactive from the Osteoarthritis Initiative (OAI) cohort of adults with or at high risk of knee OA. This proposal will utilize valuable objective longitudinal physical activity data measured on a subsample from the OAI national cohort (n=2015) to accomplish the following: Aim 1. Evaluate if a transition from inactivity to activity is related to short-term (at 2 years) function and longterm (at 4 years) function after controlling for baseline function, descriptive factors, modifiable health factors, and non-sedentary activity. This aim is relevant to clinical practice. Aim 2. Evaluate if a transition from an inactive to active state translates into differences in concurrent and subsequent health-related quality adjusted life years (QALYs). This aim is relevant to health policy design and future planning of cost-effectiveness evaluations. Aim 3. Determine the optimal physical activity threshold in the context of knee OA to maintain A) short term function and B) long term function; we will separately determine if each threshold differs by obesity subgroups. This aim is relevant to refining physical activity public health recommendations. Technical Aim. Create a prediction model crosswalk between uniaxial and triaxial physical activity measures from monitoring of n=200 OAI participants wearing both accelerometer technologies. This work will facilitate comparisons of public OAI data with future studies that use newer triaxial technology. Findings from this proposed study have important public health implications for the design of future physical activity intervention programs to improve quality of life among the 27 million U.S. adults who have OA.